An exemplary embodiment relates generally to a nonvolatile memory device and a program and read method thereof and, more particularly, to a nonvolatile memory device and a read method thereof by performing an accurate read operation by correcting a read voltage according to the retention characteristic.
Electrically programmable and erasable nonvolatile memory devices, which do not require the refresh function of rewriting data at specific intervals, are in demand.
A flash memory device, one type of nonvolatile memory devices, includes memory cells configured to have their neighboring sources and drains coupled together in series and coupled to a bit line as one unit. Each memory cell typically has a transistor structure in which a floating gate and a control gate are stacked. A memory cell array including the memory cells is directly formed within a P type well formed in a P or N type substrate. The drain of a NAND memory cell is coupled to the bit line via a selection gate, and the source thereof is also coupled to a source line via the selection gate. The control gates of the memory cells are consecutively arranged in the row directions, thus becoming respective word lines.
The operation of a NAND flash memory device is described below. A data write operation is performed from a memory cell farthest away from the bit line and sequentially to other memory cells. High voltage Vpp is supplied to the control gate of the selected memory cell, an intermediate potential is supplied to the control gate of the memory cell on the side of the bit line from the selected memory cell and the selection gate, and 0 V or the intermediate potential is supplied to the bit line depending on data. When 0 V is supplied to the bit line, the same voltage is transferred up to the drain of the selected memory cell, and so electrons are injected into the floating gate at the drain of the selected memory cell. Accordingly, the critical value of the selected memory cell is shifted in the forward direction.
In a distribution of a cell voltage of the flash memory device, the cell voltage can change or is moved with an increase in the period where data is stored. This is called a retention characteristic. During the time for which the data is stored for a long time and program and read operations are continuously performed, the cell voltage is changed or moved, which may lead to a difficulty in the read operation. That is, if a distribution of the threshold voltage of a memory cell is changed because of the retention characteristic, data can be erroneously determined and read by a read voltage fixed during the read operation, thereby causing error in the read operation.